Estimating Soil Mercury Pools and Residence Times in Coniferous and Deciduous Forest Stands.

Warmer mean annual temperatures from climate change are expected to cause coniferous stands to transition to deciduous stands in the northeastern United States. This may affect Hg sequestration in the underlying soil. We studied aboveground vegetation and Spodosols at paired coniferous and deciduous stands in the deciduous-coniferous transition zone on eight mountains in Vermont and New Hampshire, US. We found that organic horizons (forest floor) at coniferous stands accumulated more Hg (42 ± 6 g ha⁻¹) than deciduous stands (30 ± 4 g ha⁻¹). Mineral horizon Hg pools were similar for coniferous (46 ± 8 g ha⁻¹) and deciduous stands (45 ± 7 g ha⁻¹). Soil properties (C, % clay, and pH) explained 56 % of the variation in mineral soil Hg concentration when multiple regressed. Under the assumption of steady state, we calculated Hg mean residence time (MRT) in the organic and mineral horizons at coniferous and deciduous stands using a simple two-box model. We estimated Hg litterfall using late season foliage and allometric equations. Atmospheric deposition and volatilization was interpolated from existing data sets. Organic horizon MRT were longer at coniferous stands (183 ± 44 yr) than deciduous stands (65 ± 15 yr). Mineral horizons MRT were also greater for coniferous stands (386 ± 57 yr) than for deciduous stands (188 ± 27 yr). We conclude that vegetation type has an effect on Hg accumulation and retention in the organic horizons, but not in the mineral horizons.